Many manufacturers of wireless cellular telephones and handheld devices support both handset and hands free functionality in their telephone products. An external plug-in earphone/microphone is commonly used with a portable cellular telephone. The wireless cellular telephone, to support both the handset speaker and the plug-in earphone, will typically include two independent amplifiers. Two amplifiers are typically required because the handset speaker and plug-in earphone must work independently and present different loads. For example, a handset speaker load may be 8 ohms and the earphone load may be 32 ohms.
In a normal operation of using an external plug-in earphone/microphone, the handset microphone and speaker are turned off when the external plug-in earphone/microphone is in use. Since the function of using a handset and using a plug-in earphone are used separately, one amplifier remains unused when the other amplifier is active. Generally, it is difficult to use only a single amplifier because both loads cannot be permanently connected in parallel. The loads must work independently; the use of an external switch between the amplifier outputs and load generates an degradation in audio quality or increased distortion occurs.
FIG. 1 illustrates a typical prior art configuration of a switch selection circuit and amplifiers to support both a handset speaker and an external plug-in earphone. An selection switch 10 routes an audio input signal 20 to either a speaker amplifier circuit 30 or an earphone amplifier circuit 40. An amplifier circuit 30, 40 will typically include an output amplifier stage 32, 42 to drive the speaker or load and a preamplifier stage 33, 43. An amplifier circuit 30, 40 will typically include feedback loops 31, 41 to decrease distortion and improve the audio quality.
For example, in U.S. Pat. No. 6,397,087 to Kim et al. entitled “Device for Controlling the Connection of a Built-In Type Ear-Microphone for Portable Radio Terminal,” an audio processing path connected to a speaker amplifier is switched to another independent amplifier to drive the hands free ear-piece. However, Kim suffers from one or more of the following disadvantages: the use of two separate independent amplifiers, including a preamplifier and output amplifier for a handset speaker, and for a plug-in earphone which introduces redundancy in the wireless cellular telephone circuitry, additional cost, increased size and weight of the cellular telephone, and increased power consumption.